The present disclosure relates to continuous, unsupported, microporous membranes having two or more distinct, but controlled pore sizes and to processes of making and using same, more particularly to unsupported microporous membranes made from a first dope and at least one additional dope being applied directly to one another prior to the at least two dopes being quenched and to apparatus for manufacturing and processes for making such membrane.
Microporous phase inversion membranes are well known in the art. Microporous phase inversion membranes are porous solids which contain microporous interconnecting passages that extend from one surface to the other. These passages provide tortuous tunnels or paths through which the liquid which is being filtered must pass. The particles contained in the liquid passing through a microporous phase inversion membrane become trapped on or in the membrane structure effecting filtration. The particles in the liquid that are larger than the pores are either prevented from entering the membrane or are trapped within the membrane pores and some particles that are smaller than the pores are also trapped or absorbed into the membrane pore structure within the pore tortuous path. The liquid and some particles smaller than the pores of the membrane pass through. Microporous phase inversion membranes have the ability to retain particles in the size range of from about 0.01 or smaller to about 10.0 microns or larger.
Many important micron and submicron size particles can be separated using microporous membranes. For example, red blood cells are about eight (8) microns in diameter, platelets are about two (2) microns in diameter and bacteria and yeast are about 0.5 microns or smaller in diameter. It is possible to remove bacteria from water by passing the water through a microporous membrane having a pore size smaller than the bacteria. Similarly, a microporous membrane can remove invisible suspended particles from water used in the manufacture of integrated circuits in the electronics industry.
Microporous membranes are characterized by bubble point tests, which involve measuring the pressure to force either the first air bubble out of a fully wetted phase inversion membrane (the initial Bubble Point, or xe2x80x9cIBPxe2x80x9d), and the higher pressure which forces air out of the majority of pores all over the phase inversion membrane (foam-all-over-point or xe2x80x9cFAOPxe2x80x9d). The procedures for conducting initial bubble point and FAOP tests are discussed in U.S. Pat. No. 4,645,602 issued Feb. 24, 1987, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure. The procedure for the initial bubble point test and the more common Mean Flow Pore tests are explained in detail, for example, in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976) which are incorporated herein by reference to the extent not inconsistent with the present disclosure. The bubble point values for microporous phase inversion membranes are generally in the range of about two (2) to about one hundred (100) psig, depending on the pore size and the wetting fluid.
U.S. Pat. No. 3,876,738, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, describes a process for preparing microporous membranes by quenching a solution of a film-forming polymer in a non-solvent system for the polymer. U.S. Pat. No. 4,340,479, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, generally describes the preparation of skinless microporous polyamide membranes by casting a polyamide resin solution onto a substrate and quenching the resulting thin film of polyamide.
Multizone membrane offers much greater life and flow than conventional membranes while still maintaining adequate control of the thickness and pore size of each zone to ensure reliable retention. A three-zoned membrane may contain a tight or relatively small pore size zone sandwiched between two open or relatively large pore size zones. The open or relatively large pore size zones would not restrict flow but would serve to protect the tight or relatively small pore size zone from abrasion or damage, allowing it to be much thinner and still maintain integrity. Materials such as polyvinylidene fluoride (PVDF) or polyether sulfone (PES) do not require reinforcement because of their inherent strength. Scrimless membranes however, require a coating surface, such as, for example, a belt or drum to support them when cast. The die or dope applying apparatus required to cast multiple microporous membrane zones must be practically located on the same side of the coating surface and designed to control the membrane properties. Potential design constraints are discussed below. The membrane pore size of each zone can be controlled through polymer content, solvent and nonsolvent amount and temperature history of the dissolved dope. Potential constraints on these variables will be discussed below.
There is an extensive body of knowledge concerning multiple ply films and slot die technology. This prior art deals with the extrusion of films that are essentially impermeable. This prior art also discusses manufacture of both photographic film and films used in the packaging industry (e.g. food packaging). Some examples of patents, each of which are herein incorporated by reference to the extent not inconsistent with the present disclosure, disclosing multizone films are listed in the table below:
Other art involves the manufacture of microporous membranes by other techniques. Grandine provides the first practical disclosure of the manufacture of PVDF membrane. The Grandine patent (U.S. Pat. No. 4,203,847) discloses, although does not claim, that thermal manipulation of the dope will lead to a change in pore size of the resulting membrane. Surprisingly, given that nylon is a very different polymer that is dissolved in ionic organic acids rather than an organic ketone, it experiences a similar phenomenon. Grandine did not suggest a mechanism for this phenomenon to indicate that it might be general for polymers used to make membranes.
Subsequent patents relating to PVDF disclose methods for making asymmetric PVDF membrane. The Wang patent (U.S. Pat. No. 5,834,107) discloses a variety of methods to manufacture asymmetric membrane. Other patents that are related to asymmetric structure and which are cited in the Wang patent are Costar (WO 93/22034), Sasaki (U.S. Pat. No. 4,933,081), Wrasidlo (U.S. Pat. No. 4,629,563 and U.S. Pat. No. 4,774,039), and Zepf (U.S. Pat. Nos. 5,188,734 and 5,171,445).
Other prior art is the use of thermal manipulation to create distinct zones of controlled pore size with nylon membrane by Meyering et. al. (application WO99/47246, the disclosure of which is incorporated herein by reference to the extent not inconsistent with the present disclosure) applying two layers of dope against opposite sides of a support scrim after the scrim was filled with a first dope. In some applications, especially pleated cartridge filters, Nylon is an intrinsically weak material which requires the use of a scrim to function in particular applications effectively, but unreinforced or unsupported nylon is used in other applications. The presence of the reinforcing or supporting scrim requires multiple dies, one to provide dope within and to fill the scrim for the middle membrane zone and the other two dies to apply the dope for the outer two membrane zones.
Additional prior art is Degen (U.S. Pat. No. 5,500,167) which also claims a supported membrane with a porous nonwoven fibrous support wherein the two zones of the membrane are divided into zones of different pore sizes. In that case, a second dope layer to form a second zone is applied to a first dope layer in a secondary, sequential operation with the scrim partially outboard of the two finished zones.
Steadly U.S. Pat. No. 4,770,777 deals with skinned multi-layer membranes. but is not made from at least two dopes.
Another approach to joining two different membrane zones together is wet laminating wherein membranes that have been cast and quenched but not dried are joined under mild pressure and then dried together. Wet lamination is prone to delamination, which can be a particular concern if the membrane is back-flushed. As a practical matter, laminated multizone membranes tend to be thicker than single zone membranes since each zone is an independently, individually prepared membrane which included being quenched prior to being laminated together to form the multizone membranes. These prior art membranes are clearly relatively thick, as each zone of the laminated multizone membrane must be individually sufficiently thick in order to survive the membrane manufacturing process and then be joined with at least one other individual sufficiently thick membrane, individually and separately prepared, to form a multizone laminated membrane.
Slot die technology prior art on generally does not deal with the manufacture of microporous membranes nor it""s the requirements for the manufacture of microporous membranes with the exception of the Meyering et al. disclosure mentioned above.
Asymmetric membrane prior art on s does not disclose, suggest or teach independent control the properties of each zone (such as thickness or pore size) nor are the zones reliably discrete.
Finally, multizone nylon membrane prior art has conventionally required a reinforcing or supporting scrim (porous nonwoven fibrous support) in order to function in a commercial environment.
Thus, there is a need for unsupported or scrimless, multizone polymeric microfiltration membrane having at least two independent and distinct pore size performance zones progressing through the thickness of the membrane, each zone being continuously joined throughout the membrane structure. Such a multizone membrane should eliminate the reinforcing or supporting scrim while realizing the advantages of multizone filtration control. Such a scrimless multizone membrane should have at least two separate zones that are continuously joined by the molecular entanglement that occurs in the liquid state of the dope layer after the dope layer for one membrane zone is coated onto the dope layer for another membrane zone prior to phase inversion. Such a multizone scrimless membrane should be produced by a highly robust, single unit operation, with on-line pore size and zone thickness attribute control. Such a multizone scrimless membrane should be as thin as single zone membranes and thinner than prior art laminated multizone membranes. Such a multizone scrimless membrane should be relatively inexpensively and easily manufactured.
The present disclosure is directed to multizone membrane without a reinforcing or supporting scrim (nonwoven porous support), apparatuses and processes for the manufacture thereof. The scrimless or unsupported membrane may be substantially simultaneously formed into multiple (two or more) discrete zones, each with, presently preferably, a different but controlled pore size. However, it is envisioned that a multizone membrane comprising at least two adjacent zones having the same pore size might be advantageous in certain applications. Layers of dope that form the zones are applied directly to one another prior to the membrane quench such that the discrete pore structures are maintained within the separate zones but the separate zones are integrally joined.
The concept taught could be applied to nylon, PVDF, PES, PP or any membrane component wherein pore size can be controlled though dope preparation, which may include formulation of constituents or thermal manipulation prior to casting.
The present disclosure claims the process of substantially, simultaneously coating multiple fluid layers consisting of different polymer/solvent or polymer/solvent/nonsolvent solutions on to a moving self-releasing substrate and then subjecting such multiple fluid layers to a phase inversion process in, for example, a nonsolvent or solvent/nonsolvent liquid bath in such a manner as to produce an unsupported, multizone microporous membrane having multiple pore size layers. The moving casting or coating surface material is selected so the polymer solutions used for the different dope layers that form the different zones may vary from each other in terms of composition, thermal history or end-group functionality. The generic concept for the substantially, simultaneous coating of multiple fluid layers is described below.
One aspect of the present disclosure includes a process for forming a continuous, unsupported, multizone phase inversion microporous membrane having at least two zones, comprising of the acts of: operatively positioning at least one dope applying apparatus having at least two polymer dope feed slots relative to a continuously moving coating surface; cooperatively applying polymer dopes from each of the dope feed slots onto the continuously moving coating surface so as to create a multilayer polymer dope coating on the coating surface; and subjecting the multiple dope layer coating to contact with a phase inversion producing environment so as to form a wet multizone phase inversion microporous membrane.
Another aspect of the present disclosure includes a process for forming a continuous, unsupported, multizone phase inversion microporous membrane having at least two zones, comprising of the acts of: operatively positioning at least two dope applying apparatus, each having at least one polymer dope feed slot, relative to a coating surface; sequentially applying polymer dopes from each of the dope applying apparatus onto the coating surface so as to create a multilayer polymer dope coating on the coating a surface; and subjecting the sequentially applied polymer dopes to contact with a phase inversion producing environment so as to form a wet multizone phase inversion microporous membrane.
Still another aspect of the present disclosure includes a multizone, unsupported, membrane comprising: a first zone having a first pore size; and at least a second zone having a second pore size, the first and second zones being operatively connected such that the multizone membrane is continuous and does not include any support material.